When deploying wireless communication networks, there is a balance between coverage and capacity. On the one hand, a few large cells can provide great coverage but at a cost of reduced capacity. On the other hand, a scenario with many small cells creates better capacity and throughput, but may not provide the desired coverage. Hence, there is often a combination of larger cells to provide sufficient coverage with smaller cells to provide better capacity.
However, when the cells get too small, wireless devices moving in the network cause a great number of handovers which causes significant overhead. Moreover, providing coverage indoors using many small cells can be quite costly, with a radio base station for each such small cell.
One solution to this problem is to use remote radio heads, where several remote radio heads connected to the same radio base station share the same cell. In this way, a single radio base station can provide coverage in different parts of the building by placing the remote radio heads appropriately. Moreover, the wireless device can move between the coverage of different remote radio heads while staying within the same cell, thus avoiding causing handovers. The wireless device will not realize that it is served by different remote radio heads, but see it as one single cell.
When uplink signals from the remote radio heads are in the analogue domain, and these are combined in a combiner, this combination can occur coherently or non-coherently. In coherent combining, phases of the distorted desired signals are aligned prior to combining by multiplying the distorted desired signals with conjugates of respective channel estimations, increasing the signal-to-interference plus noise-ratio (SINR) of the combined signal. Several combining algorithms are available for coherent combining, including maximum-ratio combining (MRC), equal-gain combining, etc. On the other hand, for non-coherent combining, there is no phase alignment for the distorted desired signals, and the combined signal is simply the sum of all received signals. Therefore, the SINR in linear domain of the non-coherently combined signal will be equal to a linear combination, or a weighted average, of the SINRs of individual received signals.
Hence, for non-coherent combining, when one or several remote radio heads receive strong interference, i.e. having low SINR, the overall SINR is substantially degraded compared to coherent combining.